1. Field of the Invention
This invention relates to the process of preparing from carbon dioxide a mixture of dimethyl ether and methanol which are useful as clean purifying fuel or raw materials in the chemical industry. More particularly, this invention relates to the process of preparing dimethyl ether and methanol in high yield without by-products such as hydrocarbons by means of chemical conversion of carbon dioxide, which is a major pollutant of the global environment, in the presence of a mixture of catalysts comprising Cu/ZnO based catalyst and y-type zeolite catalyst having a strong acidity with the pK.sub.a value of -6.0--3.0.
2. Description of the Prior Art
Carbon dioxide is a greenhouse gas which influences the global environment in a significant manner. Recently, it has been reported that the current global warming phenomenon induced by such greenhouse gas is attributable to various adverse effects in global environment including a unusual change of weather around the world (damages in agricultural plants due to unusual change of weather, changes of the precipitation level, ecological destruction, rise in sea water level, etc.). It is presumed that the global warming phenomenon may be lethal to the existence of the mankind at some point in the future.
The concentration of carbon dioxide in air has gradually increased to the level of 353 ppm as compared to 280 ppm in the pre-industrial revolution period. In an effort to adequately preserve the global environment, more active measures should be taken on a worldwide basis so as to prevent the increasing concentration of carbon dioxide in air. In this context, there has been a conspicuous trend to regulate the use of fossil fuels or the amount of carbon dioxide generation. In light of the current industrial system, one may expect the prolonged use of fossil fuels or other chemicals, giving rise to the generation of carbon dioxide. Under such circumstances, there is an urgent need for developing a process of converting carbon dioxide into useful fuels or compounds by recycling carbon dioxide generated by the combustion of fossil fuels. According to the actual statistics, since nations in the world are dependent on the fossil fuels (e.g., petroleum or coal) to the level of 73% of the total energy source, it can be safely said that the radical reduction of carbon dioxide source is nearly impossible.
Consequently, the development of clean fuels as a substitute for the fossil fuels is suggested in order to reduce the discharged amount of carbon dioxide. Alternatively, the methods of recovering and recycling the discharged carbon dioxide are put forth as a transient or supplemental measure. In parallel with this trend, much of the research have focused on the recovering and recycling carbon dioxide around the world.
As for the method of recovering and recycling carbon dioxide, carbon dioxide may be converted into a novel chemical material through chemical, biological, optical or electrochemical methods. With respect to above methods, the related studies have been numerously reported in the past. In order to recover a large amount of carbon dioxide discharged into air for the purposes of the consequent reaction, the pertinent reaction rate should be extremely rapid, and the final products should embrace a wide scope of market demand. Among these methods, the chemical conversion process of carbon dioxide has been reported to be most appropriate, and there is a market demand for converting carbon dioxide into widely applicable basic materials in the chemical industry or energy-based products instead of a small amount of chemical products.
As for the mass scale synthesis of the basic materials such as methanol, hydrocarbon, etc. from the chemically stable carbon dioxide, the reduction method using hydrogen in the presence of catalyst may be easily developed based on the current technology. Thus, the research on the topic of actually synthesizing methanol from carbon dioxide and hydrogen have been actively carried out, and there have been a great number of studies reporting the use of the catalyst system containing Cu/ZnO as basic constituent.
In the case where methanol is synthesized by chemical conversion of carbon dioxide, the copper-based catalysts are employed, and two reactions as shown in the following schemes 1 and 2 occur on a parallel basis.
Scheme 1 EQU CO.sub.2 +3H.sub.2.fwdarw.CH.sub.3 OH+H.sub.2 O
Scheme 2 EQU CO.sub.2 +H.sub.2.fwdarw.CO+H.sub.2 O
As noted in the above schemes 1 and 2, these simultaneous reactions are subject to thermodynamic limit, and the conversion rate of carbon dioxide into methanol and the selectivity among the reaction products of the generated methanol have constant equilibrium values as function of temperature and pressure. When methanol is synthesized from carbon dioxide, the low conversion rate of carbon dioxide into methanol requires the recycling of carbon dioxide in a large amount, and this is not preferable in the manufacturing process.
To resolve this matter, much of the endeavors have been made so as to overcome the low conversion rate of carbon dioxide owing to such thermodynamic limit, i.e., a simultaneous execution of the reaction designed to synthesize methanol from carbon dioxide and the reaction of converting methanol, so formed, into the second dimethyl ether, as shown in the following scheme 3. As such, by reducing the amount of methanol within a reactor, the thermodynamic limit is alleviated, which in turn leads to the improvement of the conversion rate of carbon dioxide and simultaneous generation of methanol and dimethyl ether.
Scheme 3 EQU 2CH.sub.3 OH.fwdarw.CH.sub.3 OCH.sub.3 +H.sub.2 O
Dimethyl ether, an intermediate generated in the MTG (methanol to gasoline) process of preparing various hydrocarbons from methanol, has a high potential use as a basic material in the chemical industry in addition to being a highly effective clean fuel. Presently, there is a possibility that dimethyl ether may be substituted as a clean fuel for the internal combustion engine. As such, the reaction for generating dimethyl ether directly from carbon dioxide may provide two advantages as follows: 1) the process conversion rate of carbon dioxide may be improved, and 2) the utility of dimethyl ether, so formed, is not inferior to that of methanol. Furthermore, since there is a significant difference in boiling point between dimethyl ether and methanol, the separation is easily obtained for manufacturing thereof.
J. L. Dubois et al. discloses a process for preparing dimethyl ether and methanol from carbon dioxide and hydrogen using a catalyst consisting of Cu/ZnO/Al.sub.2 O.sub.3 catalyst and Y-type zeolite [Chem. Lett., 1115 (1992)]. According to this method, HY-type zeolite for commercial use was employed without modification. However, according to the studies by the inventors herein, HY-type zeolite with extremely strong acidity (pKa.gtoreq.-8.2) may result in generating hydrocarbon by-products such as methane, ethane, propane, etc. The generated hydrocarbons as by-product belong to the alkane group with low molecular weight and have recognized disadvantages in that a) their utility is less significant as reaction products, b) it is not easy to separate some of the hydrocarbons from carbon dioxide during the separation process of the reactions products after said reactions. Therefore, even though a small amount of hydrocarbons is generated, the recycling process of unreacted carbon dioxide causes the continual accumulation of by-products (e.g., hydrocarbon, etc.) within the reaction gas, whereby affecting the process steps in a negative manner.